On the Demystification of HPbI3 and the Peculiarities of the Non-innocent Solvents H2O and DMF

Detailed investigations by XRD reveal that the precursor HPbI3 that was obtained by reaction of aq. conc. hydroiodic acid HI and PbI2 in DMF is (CH3)(2)NH2PbI3. (CH3)(2)NH2+ is formed by solvent reaction as already described in the literature but not properly assigned. Attempts to synthesize HPbI3 by gas phase reaction of PbI2 with aq. conc. HI yielded light-yellow crystals of the oxonium salt H18O8Pb3I8 (Pbam, Z = 2, a = 10.075, b = 30.162, c = 4.5664 angstrom). The crystal structure of H18O8Pb3I8 consists of trimeric ribbons of edge-sharing PbI6 octahedra. These ribbons [Pb3I8](2-) are separated by protonated fragments of crystalline ice [H18O8](2+) or (H2O)(6)(H3O+)(2). H18O8Pb3I8 can also be precipitated from PbI2 and aq. conc. HI. H18O8Pb3I8 is not stable at room temperature but transforms within a few days to light-yellow (H3O)(2x)(H2O)(2-2x)Pb1-xI2 with x approximate to 0.23 (R3m, Z = 3, a = 4.5554, c = 29.524 angstrom). The crystal structure represents a CdCl2-type layer structure with H2O/H3O+ in between. Charge compensation is achieved by Pb2+ vacancies. Via topotactic reaction (H3O)(2x)(H2O)(2-2x)Pb1-xI2 releases H2O/HI and forms crystals of the pristine PbI2. All steps were characterized by P-XRD, IR/Raman spectra, and UV/Vis spectra. H18O8Pb3I8 acts as a precursor for the synthesis of MAPbI(3) because the reaction with gaseous CH3NH2 yields MAPbI(3), so it can mimic a composition HPbI3.